1. Field of the Invention
The present invention generally relates to optical scanning apparatuses and, more particularly, to an optical scanning apparatus in which adverse effects caused by sag and ghost are reduced.
2. Discussion of the Background
An optical scanning apparatus is well known for its application in digital copiers and optical printers. Generally a polygon mirror is used in an optical scanning apparatus to deflect a light beam. Because a center of rotation of a polygon mirror does not coincide with a deflecting and reflecting surface, the position of reflection of a beam at the deflecting and reflecting surface varies as the polygon mirror rotates. Such a phenomenon is known as a sag.
If the optical system of an optical scanning system is designed without allowing for a sag, the resultant performance of the optical scanning apparatus in terms of curvature of field and the like is unfavorable such that the performance level expected from the design is not obtained.
Defining the line of the direction of a chief ray of the polygon-deflected beam perpendicular to the scanned surface as a reference line, a sag is asymmetrical with respect to the reference line unless no correction measures are taken. Japanese Laid-Open Patent Application No. 6-230308 discloses an approach to symmetrize the sag by configuring a scanning and imaging lens for focusing the deflected beam on the scanned surface to be asymmetrical with respect to the reference line. However, it is technically difficult and costs considerably to manufacture a scanning and imaging lens which is asymmetrical in configuration.
Japanese Laid-Open Patent Application No. 4-110818 details reducing the degree of asymmetry in curvature of field and distortion aberration by configuring a lens to have an incidence surface formed as a toroidal surface and an exit surface formed as a spherical surface, and by designing the toroidal surface and the spherical surface to be relatively shifted from each other.
While such a method is helpful to reduce the degree of asymmetry in curvature of field and distortion aberration, flexibility in correcting curvature of field suffers because the lens surfaces are formed as a normal toroidal surface and a spherical surface.
A description will now be given of another related art drawback addressed by the present invention.
Many optical scanning apparatuses are designed to operate at a high optical usage efficiency in order to achieve high-speed optical scanning. As the optical usage efficiency and the light intensity of a beam spot formed on the scanned surface become higher, a phenomenon called ghost presents a problem. A ghost occurs when the deflected beam irradiating the scanned surface to form a beam spot is reflected by the scanned surface to the polygon mirror, reflected then by the deflecting and reflecting surface of the polygon mirror and irradiating the scanned surface as a ghost beam.